The non-mammalian thrombocyte is the physiological equivalent of the mammalian platelet. Although it is not aggregated by ADP, it does aggregate with thrombin and collagen. In the case of avian thrombocytes, 5-hydroxytryptamine acts as the mediator of aggregation in a manner similar to ADP in platelets. The present program is involved with determining whether 5-hydroxytryptamine occurs in granules along with adenine nucleotides as is the case in the mammalian platelets. We are also examining whether prostaglandins are formed during aggregation of avian thrombocytes. Aggregation inducing substance is released from amebocytes of the horseshoe crab Limulus polyphemus. These same cells also contain the system responsible for the protein gel that acts as a defense against invasion by foreign organisms. We plan to determine if these two systems are separate or inter-related. We also propose to examine the relation of calcium influx to the release reaction.